This invention relates to adhesive materials useful in the formation of circuit materials, circuits, and multi-layer circuits.
As used herein, a circuit material is an article used in the manufacture of circuits and multi-layer circuits, and includes circuit laminates, bond plies, resin coated conductive layers, free films, and cover films. Circuit materials are formed from dielectric materials that can include thermosetting and thermoplastic polymers. The polymers may be combined with fillers such as silica to adjust the dielectric or other properties of the polymers. The adhesive layer in a circuit laminate, bond ply, build-up film, resin covered conductive layer, free film, or cover film is designed to soften or flow during manufacture of the circuit laminate or multi-layer circuit but not in use of the circuit. Dielectric substrate materials that are not reinforced with a fibrous web are often referred to as “flexible,” and are used to make flexible (sometimes referred to as “flex”) circuit materials and circuits. Flexible circuit materials are thinner, and are used in a wide variety of advanced electronic applications.
A circuit laminate is a type of circuit material that has a conductive layer fixedly bound to a dielectric substrate layer. Double clad laminates have two conductive layers, one on each side of the dielectric substrate. Patterning a conductive layer of a laminate, for example by etching, provides a circuit layer, and thus a circuit. Multi-layer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multi-layer circuits are formed by laminating one or more circuits together using bond plies, free films and, in some cases, adhesive-coated conductive layers, in proper alignment using heat and/or pressure. The bond plies or free films are used to provide adhesion between circuits and/or between a circuit and a conductive layer, or between two conductive layers. In place of a conductive layer bonded to a circuit with a bond ply, the multi-layer circuit may include a resin coated conductive layer bonded directly to the outer layer of a circuit. In such multi-layer structures, after lamination, known hole forming and plating technologies may be used to produce useful electrical pathways between conductive layers.
Epoxy-based adhesive systems are commonly used in circuit laminates, bond plies, free films, resin covered conductive layers, and cover films for flexible and rigid-flex circuits as well as for build-up layers in device packaging applications. Epoxy-based adhesive systems (hereinafter “epoxy adhesives”) generally contain an epoxy resin, a nitrile rubber, one or more flame retardants, and small amounts of other resins, additives, and cure agents. Use of a rubber additive, in particular a nitrile rubber, is essential to provide both flexibility and sufficient bond to the copper conductive layers. Nitrile rubbers are copolymers of acrylonitrile and one or more conjugated diolefin monomers having 3 to 5 carbon atoms, for example 1,3-butadiene, isoprene, 1,3-pentadiene and the like. Nitrile rubbers are preferred because they have excellent compatibility with epoxy, as well as other beneficial properties such as good thermal and chemical resistance, high polarity, and the like. The nitrile rubbers often have functional terminal groups, including carboxyl terminations (CTBN), amine terminations (ATBN), and epoxy terminations (ETBN), to allow crosslinking with the epoxy, thereby providing enhanced thermal and chemical resistance.
High molecular weight grade rubbers (such as Nipol 1072 from Zeon Chemicals) are preferred because the lower flow prevents displacement from the stack (“squeezing out”) during the lamination process. However, it has been found by the inventors hereof that the combination of epoxy resin and nitrile rubber leads to certain problems associated with manufacture and use of the circuit materials and the circuits produced therefrom. One such problem is the potential for dendritic growth to occur under DC (direct current) bias at elevated temperature and humidity levels. Minimizing such dendritic growth would provide flexible circuit materials, circuits, and multi-layer circuits with even greater stability under conditions of high heat and humidity.
Another drawback of current epoxy adhesives used for flex circuits is their relatively short shelf life. During storage over time the flow characteristics of the partially cured (“B-staged”) epoxy adhesive changes. This requires circuit fabricators to manage their inventory of materials carefully. One of the primary causes of the short shelf life is reaction between the carboxyl group on the high molecular weight nitrile rubber and the epoxy resins. This reaction occurs slowly at room temperature, but because of the high molecular weight of the rubber, even a small number of crosslinks will significantly reduce flow of the adhesive, which can cause problems during circuit fabrication. Extending the shelf life of the adhesive by eliminating the epoxy-carboxyl reaction in the adhesive would greatly enhance the usefulness of the products made using this adhesive.
In addition, the worldwide trend to eliminate hazardous materials in the construction of electronic devices has necessitated the removal of lead (Pb) from solder. The Pb-free solders currently in use melt at temperatures 30 to 40° C. higher than the standard tin-lead (Sn—Pb) solder they replace. Actual Pb-free solder reflow processes expose the circuits to temperatures as high as 260° C., and material qualification testing often stresses the materials at 288° C. Increasing the ability of the material systems to survive the higher temperature processes requires an adhesive with superior thermal resistance.
There accordingly remains a need in the art for epoxy adhesive systems for flexible circuits that have one or more of lower dendritic growth under conditions of high heat and humidity, are capable of surviving thermal exposures required for Pb-free solder processes, and/or are more stable as a b-staged adhesive. All of these attributes are preferably present without comprising the other advantageous properties of the epoxy systems, such as good adhesion and good processability.